let's assume littleEndian in JS backend

[ringabout]

Most systems are little endians(especially for JS, it's more meaningful to assume little endians.

Ref https://hacks.mozilla.org/2017/01/typedarray-or-dataview-understanding-byte-order

So, in summary, here’s what we’ve learned:
It is safe to assume systems to be little-endian.

[ringabout]

getEndians for C/JS/VM backend

when defined(js):
  import std/jsbigints

  type
    ArrayBuffer* = ref object of JsRoot
    Float64Array* = ref object of JsRoot
    Uint32Array* = ref object of JsRoot
    BigUint64Array* = ref object of JsRoot

  func newArrayBuffer*(n: int): ArrayBuffer {.importjs: "new ArrayBuffer(#)".}
  func newFloat64Array*(buffer: ArrayBuffer): Float64Array {.importjs: "new Float64Array(#)".}
  func newUint32Array*(buffer: ArrayBuffer): Uint32Array {.importjs: "new Uint32Array(#)".}
  func newBigUint64Array*(buffer: ArrayBuffer): BigUint64Array {.importjs: "new BigUint64Array(#)".}

  func `[]`*(arr: Uint32Array, i: int): uint32 {.importjs: "#[#]".}
  func `[]`*(arr: BigUint64Array, i: int): JsBigInt {.importjs: "#[#]".}
  func `[]=`*(arr: Float64Array, i: int, v: float) {.importjs: "#[#] = #".}

  proc hasJsBigInt*(): bool =
    asm """`result` = typeof BigInt != 'undefined'"""

when defined(js):
  type
    Uint8Array* = ref object of JsRoot

  func buffer(p: Uint32Array): ArrayBuffer {.importjs: "#.buffer".}


  func newUint32Array*(arr: openArray[uint32]): Uint32Array {.importjs: "new Uint32Array(#)".}

  func newUint8Array(p: ArrayBuffer): Uint8Array {.importjs: "new Uint8Array(#)".}

  func `[]`*(arr: Uint8Array, i: int): uint8 {.importjs: "#[#]".}


when defined(js):
  when defined(nodejs):
    {.emit: "const _nim_nodejs_os = require('os');".}

    proc endianness(): cstring {.importjs: "_nim_nodejs_os.endianness()".}

    template getEndiansImpl(): Endianness =
      if endianness() == "LE":
        littleEndian
      else:
        bigEndian
  else:
    template getEndiansImpl(): Endianness =
      let a = newUint32Array([0x12345678'u32])
      let b = newUint8Array(a.buffer)
      if b[0] == 0x78:
        littleEndian
      else:
        bigEndian

template getEndiansImpl(): Endianness =
  when defined(i386) or defined(alpha) or defined(powerpc64el) or
      defined(ia64) or defined(amd64) or defined(mipsel) or defined(arm) or
      defined(arm64) or defined(avr) or defined(msp430) or defined(riscv32) or
      defined(riscv64) or defined(esp) or defined(wasm32) or defined(mips64el):
    littleEndian
  elif defined(m68k) or defined(powerpc) or defined(powerpc64) or defined(sparc) or
      defined(hppa) or defined(mips) or defined(sparc64) or defined(mips64):
    bigEndian
  else:
    littleEndian

proc getEndians(): Endianness {.inline.} =
  when nimvm:
    getEndiansImpl()
  else:
    when defined(js):
      result = getEndiansImpl()
    else:
      result = cpuEndian

static: echo getEndians()
echo getEndians()

[timotheecour]

LGTM.
future work:

• cross reference system.cpuEndians and std/endians (help welcome @konsumlamm)
• add tests/stdlib/tendians
• add tests for cpuEndian in c/js/vm
• add getEndian in std/endians as shown in let's assume littleEndian in JS backend #16886 (comment) or make js code correct on little + big endian timotheecour/Nim#515